1. Field of the Invention
The present invention relates to a method, in a water-base system, of at least partially avoiding an expected viscosity decrease or of providing a viscosity increase upon a temperature increase, by using a system containing as viscosity-adjusting agent a nonionic water-soluble cellulose ether and a water-soluble ionic surface-active compound.
2. Background of the Art
Water-soluble polymers in most cases give rise to a substantial viscosity increase when dissolved in water. A large number of water-base systems therefore contain polymers and viscosity-adjusting agents. Also small additions, e.g. below 1% by weight, may cause considerable changes in the viscosity of the aqueous solution and even give rise to gelation. The viscosity of an aqueous solution of a polymer generally is dependent on concentration and temperature. Normally, the viscosity increases upon an increase of the polymer concentration or upon a temperature drop. Examples of polymers exhibiting such a temperature dependence are sodium alginate, polyvinyl alcohol and tragacanth.
There are many types of gel-forming water-soluble polymers. In most of these polymers, gelation occurs upon a temperature drop. Some polymers exhibit in aqueous solution a so-called turbidity point or flocculation temperature, which means that the polymer precipitates when the temperature is raised above a critical temperature. Known examples of such polymers are nonionic cellulose ethers. The normal behaviour of nonionic cellulose ethers is that the viscosity is reduced when the temperature is increased above room temperature. The reduction continues until flocculation temperature is attained. At this temperature, the solution starts becoming turbid and if the concentration of the polymer is sufficiently high, a viscosity increase can be observed. The process is reversible.
Protein solutions are somewhat exceptional in respect of temperature dependence. They often exhibit irreversible processes depending on so-called denaturation of the proteins when the temperature is raised. This in turn may give rise to irreversible gelation upon a temperature increase.
Thus, it can be concluded that the viscosity of a polymer solution is reduced upon a temperature increase, although certain polymers exhibit a substantial viscosity increase at their flocculation temperatures.
In many applications it is desirable to have access to water-base polymer solutions which exhibit a smaller viscosity decrease upon a temperature increase than current polymers or even a maintained or increased viscosity before gelation (viscosity increase associated with phase separation) possibly occurs.
Such properties are valuable in ceramic processes when aqueous mixtures should be extruded and the product thereafter heated with unaltered geometry. To this end, a constant or an increasing viscosity with increasing temperature is needed in order that the structure of the extruded product should not collapse. This is also the case in several applications within the food industry where extrusion is often carried out with subsequent heating. In deep-frying, it may sometimes be desirable to increase the viscosity when the temperature is raised so as to reduce the penetration of frying-oil into the food product. Other fields of use are liquid compositions for therapeutic or cosmetic treatment of man and animal where it is desirable in many cases that the viscosity of the compositions be raised, possibly followed by gelation upon heating to body temperature. Further examples are found in suspension polymerisation when it is desirable to provide stabilisation of monomer-containing droplets at increased temperatures.
Canadian Patent Specification 1,072,413 discloses a method for preparing water-base polymer compositions which are liquid at room temperature, but lose their fluidity and form a gel upon a temperature increase. This is achieved by using a gel-forming component in the form of a special type of block polymer of ethylene oxide and propylene oxide, optionally in combination with a modifying polymer component.